The present application claims priority to Application No. 101 55 391.9, filed in the Federal Republic of Germany on Nov. 10, 2001, which is expressly incorporated herein in its entirety by reference thereto.
The present invention relates to a method of charging and discharging a piezoelectric element.
Piezoelectric elements have a characteristic of contracting or expanding as a function of a direct voltage applied to them or of a direct voltage established across them. The practical implementation of actuators using piezoelectric elements is suitable in particular when the appropriate actuator has to perform quick and/or frequent movements. Among other things, the piezoelectric element is used in fuel injectors for internal combustion engines. For certain applications it is necessary that the piezoelectric element be able to be brought to different expansions or if needed to varying expansions as precisely as possible, for example, when the piezoelectric element is used as an actuator in a fuel injection system. Through direct or indirect transmission to a control valve, different expansions of the piezoelectric element correspond to the displacement of an actuator, like a nozzle needle for example. The displacement of the nozzle needle results in the opening of injection orifices. The duration of the opening of the injection orifices corresponds to a desired injected fuel quantity as a function of a free cross-section of the orifices and an applied pressure. The transmission of the expansion of the piezoelectric element to the control valve is differentiated into two basic transmission modes. In the first, direct, transmission mode, the nozzle needle is moved directly by the piezoelectric element via a hydraulic coupler. In the second transmission mode, the movement of the nozzle needle is controlled by a control valve which is triggered by the piezoelectric element via a hydraulic coupler. The second, the indirect transmission mode, corresponds to the main industrial application and is the basis for further explanations. The hydraulic coupler has essentially two characteristics; first, the reinforcement of the stroke of the piezoelectric element, and second, the decoupling of the control valve from a static thermal expansion of the piezoelectric element. The hydraulic coupler transmits the elastic deflection of the piezoelectric element to the control valve within a control cycle. In order to function accurately, the coupler must be sufficiently filled with a fluid. In each control cycle, which includes a charging operation, a holding operation, and a discharging operation, a portion of the fluid being present in the coupler is pressed out via leak gaps.
In particular during the charging and holding operation, where the piezoelectric element is charged to a certain voltage, the hydraulic coupler undergoes a certain deflection and moves a valve element of the control valve from a first seat to a second seat. To ensure an accurate opening of the nozzle needle, the valve element of the control valve, as a rule, must be in contact with the second seat and must seal against a high pressure applied in a rail chamber. If this is not the case, as a rule, an unintentional deflection of the actuator occurs due to the pressure changes of a control space above the actuator and thus resulting in an imprecise injected fuel quantity. Due to existing leakage losses at the leak gaps of the hydraulic coupler, a decrease of coupler pressure results during the holding operation. The level of a setpoint direct voltage of the piezoelectric element, applied within the charging operation, is reduced in response to the decrease in the coupler pressure. If the coupler pressure drops to a certain level, then the valve element of the control valve can no longer be held in the second seat and leakages occur in the second seat of the control valve. Thus, after a short time, leakages in the sealing area of the second seat occur, in particular at high pressures in the rail chamber. This results in a pressure change inside the control space. This pressure change results in an unintended actuating movement of the actuator and thus to an imprecise injected fuel quantity. The decrease in the voltage applied to the piezoelectric element during the holding phase indicates a leakproof condition of the control valve on the second seat and thus a correct function of the injector.
The method according to the present invention provides an actuating movement of the actuator that is changed as a function of the level of the voltage applied to the piezoelectric element, a decrease in the level of the voltage applied during a holding phase being compensated for by at least one additional transmission of electric charge carriers from the direct voltage source to the piezoelectric element. Thereby the pressure in the coupler is elevated again to a level where a leakproof condition in the second seat may be ensured.
In an exemplary embodiment of the present invention, after a reduction in the setpoint direct voltage of the piezoelectric element within the holding operation of the control cycle, a retransmission of electric charge carriers from the direct voltage source to the piezoelectric element takes place at a predefined point in time. This method results in the compensation for the pressure drop in the coupler due to retransmission of electric charge carriers and in the prevention of the movement of the actuator at a wrong instant, and thus in a reliable control and an accurate regulation of the fuel quantity being injected.
The present invention is explained below in greater detail using an exemplary embodiment with reference to the drawing.